CA1129600A - Flexible polyalkenyl aromatic polymeric foamed sheet - Google Patents
Flexible polyalkenyl aromatic polymeric foamed sheetInfo
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- CA1129600A CA1129600A CA355,505A CA355505A CA1129600A CA 1129600 A CA1129600 A CA 1129600A CA 355505 A CA355505 A CA 355505A CA 1129600 A CA1129600 A CA 1129600A
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Abstract
ABSTRACT OF THE DISCLOSURE
The invention relates to a low density foamed polymeric sheet comprising a polyalkenyl aromatic polymer having dispersed therein about 0.1 to 5.0% of a polyolefin resin, based on said polymer, said foamed sheet having multiaxial flexibility. A process for preparing said foamed sheet is also disclosed.
The invention relates to a low density foamed polymeric sheet comprising a polyalkenyl aromatic polymer having dispersed therein about 0.1 to 5.0% of a polyolefin resin, based on said polymer, said foamed sheet having multiaxial flexibility. A process for preparing said foamed sheet is also disclosed.
Description
36~
A FLEXI~LE POLYALKENYL A~O~TIC POLYMERIC FOAMED SHEET
BACKGROUND OF TH~ I~VE.~TION
.... ~
Polyalkenyl aromatic polymeric foamed sheets are known to the ar-t. Such sheets are known to be fragile and, hence, limit their acceptance in the art.
Heretofore, such sheets have been generally foamed with blowing agents that have a relatively high solubility in the polyalkenyl aromatic polymer-such 2S fluorocarbons or petroleum hydrocarbons, e.g. pen1:ane and the like.
Such sheets retain such blowing agents during and after manufacture. As such blowing agents migrate from the sheet ~he physical properties change with time and the sheet gradually becomes brittle, as the plasticizing ef~ects of the blowing agent is lost. Since such blowing agents have an equilibrium solubility in the polyalkenyl aromatic polymers, varying levels of the blowing agent are retained which caused the sheet to vary in physical prop- `
erties. Because of the solubility of such blowing agents in the polymer small and uniform cell size distribution has been difficult to produce without using incompatible nucleating agents that can degrade foam sheet properties.
U. S. Patent 3,770,666 discloses such foams blown with fluorocarbons with solid nucleating agent to produce fine cell size, wherein about 50% of the blowing agent is retained in ,he cells. U. S. Patent 3,231,524 discloses such foams blown with hydrocarbons or fluorocarbons using a styrene/maleic anhydride copolymer as nucleating agent.
~2~ C-08-12-0~03 . S. Patent 3,084,126 discloses polyalkenyl aromatic foamed polymers using hydrocarbons with silicates as nucle-ating agents. U. S. Patent 3,658,973 discloses a process for extruding foamed polyalkenyl aromatic polymers using fluorocarbons and hydrocarbons with carbon dioxide as a nucleating agent. U. S. Patent 2,941,964 discloses the use of hydrocarbons and a carbon dioxide liberacing agent as a nucleating agent with data to show that carbon dioxide liberating agents alone fail to produce low density foams.
It is the objective of the present invention to pro-vide a low density foamed polymeric sheet, based on a poly-alkenyl aromatic polymer, that has multiaxial flexibility.
Another objective is to provide an improved process wherein polyalkenyl aromatic polymers can be formed into foamed sheets tha~ have multiaxial flexibility as extruded or after loss of blowing agent.
SUMMARY OF THE INVENTION
The present invention relates to a low density foamed polymeric sheet comprising a polyalkenyl aromatic polymer having dispersed therein about 0.1 to 5.0% of a polyolefin resin, based on said polymer, said foamed sheet having multiaxial flexibility.
The present invention also relates to a process for preparing a foamed polymeric sheet by extruding a composi-tion comprising:
(A~ a polyalkenyl aromatic polymer, (B) a volatile blowing agent, and ~C) a nucleating agent, as a melt under pressure through a sheet die into a zone of low pressure to permit foaming of said composition, the improvement which comprises having present in said composition about 0.1 to 5~ by weight of a polyolefin resin based on said alkenyl aromatic polymer, providing a foamed sheet having multiaxial flexibility.
~ILZ96~
PREFERRED EMBODIME~TS
The polyalkenyl aromatic polymers are prepared from alkenyl aromatic monomers selected from the group consist-ing of styrene, alpha methyl styrene, aralkyl styrene, e.g., ortho, para and meta substituted styrenes with alkyl groups such as methyl, ethyl, etc.; arhalostyrenes such as chloro and bromostyrenes, aralkylhalostyrenes, e.g., para-methyl~ortho, chloro or bromo styrene. Such polymers have a molecular weight of about 100,000 to 600,000 The polyolefin resins are those prepared by conven-tional polymerization having a molecular weigh~ of at least about 20,~00, pre~erably ~0,000 to 500,000 and a melt index (gm/10 mm) of about 0.1 to 55, preferably about 0.15 to 15 with a density of about 0.890 to .967 gms/cu.cm. Such resins are available from Union Carbide, New York, N. Y., and Northern Petrochemical Inc., Des Plaines, Illinois.
Conventionally, the nucleating agents are made up of -two materials which react to form carbon dioxide and water.
The two materials are normally used in approximately equivalent amounts. As the carbon dioxide libera~ing materials there can be used ammonium, alkali and alkaline earth carbonates or bicarbonates, e.g., ammonium bicarbon-ate, sodium bicarbonate, sodium carbonate, potassium bi-carbonate, calcium carbonate. The other material is an a~id or acid-reacting salt, preferably solid~ which is sufficiently strong to liberate the carbon dioxide from the carbonate cr bicarbonate. Generally, the acid has at least 3.0 milliequivalents of acidic hydrogen, and prefer-ably at least 10.0 miliequivalents, per gram. The acid can be organic or inorganic. Suitable acidic materials include boric acid, sodium dihydrogen phosphate, fumaric acid, malonic acid, oxalic acid, citric acid, tartaric acid~ potassium acid tartrate, chloro-acetic acid, maleic acid, succinic acid and phthalic acid. In place of the anhydrous acids or salts there can be used the solid hy-drates, e.g., oxalic acid dihydrate and citric acid mono-~2~6~) hydrate.
The nucleating agents can be solid inert materials such as talc, wax or hydrated salts or silicates used in finely divided form. The nucleating agents are used in amounts of about 0.2 to 10% by weight based on the poly-alkenyl aromatic polymer, preferably about 0.2 to 2% by weight is used. The nucleating agents are conventionally dry blended with the polymer prior to extrusion.
As the volatile blowing agent liquid, there can be used aliphatic hydrocarbons boiling between 10 and 100C.
and preferably between 30 and 90C., e.g., petroleum ether (containing primarily pentane or hexane or a mixture of these hydrocarbons), pentane, hexane, isopentane, cy-clohexane, cyclopentane, pentadiene and neopentane. Other volatile liquids include methanol, ethanol, methyl ace-tate, ethyl acetate, butane, acetone, methyl formate, ethyl formate, dichloroethylene, perchloroethylene, dichloro-tetrafluoroethane, isopropyl chloride, propionaldehyde, diisopropyl ether, dichlorodifluoromethane, a mixture of pentane with 5 to 30% of methylene chloride or other vola-tile lower halogenated hydrocarbon. Trichlorofluoro-methane, methyl chloride, ethyl chloride and other fluoro-carbons of khe Freon type may be used. Gases such as CO2 and N2 have also been found to be suitable blowing agents.
The blowing agent is used in amounts of about 2 to 20% by weight based on the polyalkenyl aromatic polymer.
One suitable apparatus for extrusion of said poly-alkenyl aromatic polymers containing carbon dioxide dis-persed therein under pressure through a die into a zone of low pressure is disclosed in U. S. Patent 3,451,103.
The extrusion apparatus consists of an extruder (preferably a single screw extruder) which contains three separate functional zones or sections. The first or plas-ticating zone of the extruder melts and delivers the melted resin to the second zone at a high temperature and pressure. The structure and design of the screw in the first zone may take a wide variety of forms, but typically 96~) consists of a constant pitch screw which increases in root diameter in the downstream direction. Heating means are usually included in the first zone to assist in melting the resin. If desired, the first zone may consist of two S elements, as for example by having a plasticating extruder arranged in tandem with the plasticating zone of a second extruder and delivering melted resin thereto.
The screw in the second zone may take a wide variety of forms, but usually is a constant pitch screw which has a constant root diameter. In addition, the root diameter in the second zone is usually either identical with or slightly smaller than the root diameter at the discharge end of the first zone. The second or injection zone is provided with specially designed means for injecting liquid substances into the melted resin. It is preferred to employ a plurality of such injection means and to have them symmetrically disposed about the chamber wall. The injection means employed are capable of injecting the liquid into the extruder at a pressure substantially higher than the pressure of the mel-ted resin, e.g., at a pressure of at least about 500 p.s.:i. higher than the pressure of the melted resin. The injection means also include an element adapted to seal the liquid delivery orifice when li~uid is not being injected into the melted resin. This feature prevents the melted resin from flow-ing into and plugging the orifice of the injection means.
The sealing element preferably consists of ta) a discharge orifice whose feed inlet terminates in a valve face, (b) a cooperatively functioning valve which is adapted to seat against and seal the valve face of the orifice, (c) a first and fixed pressure means acting upon and urging the valve into seated relationship with the valve face, and (d) a second pressure means acting upon the valve and urging it out of seated relationship with the valve face;
said second pressure means being responsive to and actu-ated by the pressure of the liquid within the injection means. To prevent melted resin ~rom flowing into the in-jection means, the fixed pressure means urging the valve ~2~6~
to seat against and seal the valve face should be preset to a pressure above that developed within the resin in the second zone of the extruder.
The third zone of the extruder performs two functions.
First, the pressure on the melted resin is increased to the level required to express the resin through the die.
Second, the melted resin is cooled (or in some circumstances heated) to substantially the temperature at which it will leave the die. To properly cool (or heat) the resin, at least the aft section of the third zone should include external heat transfer means. Depending upon the length of the second zone of the extrusion apparatus, it is sometimes desirable to maintain the mixture of melted resin and liquid at a relatively high temperature in the fore section of the third zone. In this event, external heating means may be provided to heat the chamber wall oE the fore section of the third zone.
In addition, the root diameter of the screw may be increased in the fore section of the third zone so that frictional heat will be developed within the resin. In this event, however, the root diameter is preferably subsequently decreased in the aft section of the third zone.
The die affixed to the extrusion apparatus may be of any design presently used in extruding thermoplastic resins.
Scores of suitable dies are known and reported in the art.
The three zones described can be separate extruders as practiced in the art.
The extrusion can be carried out at tempera-tures of about 135 to 160C. for the extrusion temperature at the die. Pressures for extrusion generally are greater than 30 1000 psi and range 1700 to 3700 psi with the blowing agent being injected at pressures at least about 500 psi higher than the pressure of melted resin in the extruder. About
A FLEXI~LE POLYALKENYL A~O~TIC POLYMERIC FOAMED SHEET
BACKGROUND OF TH~ I~VE.~TION
.... ~
Polyalkenyl aromatic polymeric foamed sheets are known to the ar-t. Such sheets are known to be fragile and, hence, limit their acceptance in the art.
Heretofore, such sheets have been generally foamed with blowing agents that have a relatively high solubility in the polyalkenyl aromatic polymer-such 2S fluorocarbons or petroleum hydrocarbons, e.g. pen1:ane and the like.
Such sheets retain such blowing agents during and after manufacture. As such blowing agents migrate from the sheet ~he physical properties change with time and the sheet gradually becomes brittle, as the plasticizing ef~ects of the blowing agent is lost. Since such blowing agents have an equilibrium solubility in the polyalkenyl aromatic polymers, varying levels of the blowing agent are retained which caused the sheet to vary in physical prop- `
erties. Because of the solubility of such blowing agents in the polymer small and uniform cell size distribution has been difficult to produce without using incompatible nucleating agents that can degrade foam sheet properties.
U. S. Patent 3,770,666 discloses such foams blown with fluorocarbons with solid nucleating agent to produce fine cell size, wherein about 50% of the blowing agent is retained in ,he cells. U. S. Patent 3,231,524 discloses such foams blown with hydrocarbons or fluorocarbons using a styrene/maleic anhydride copolymer as nucleating agent.
~2~ C-08-12-0~03 . S. Patent 3,084,126 discloses polyalkenyl aromatic foamed polymers using hydrocarbons with silicates as nucle-ating agents. U. S. Patent 3,658,973 discloses a process for extruding foamed polyalkenyl aromatic polymers using fluorocarbons and hydrocarbons with carbon dioxide as a nucleating agent. U. S. Patent 2,941,964 discloses the use of hydrocarbons and a carbon dioxide liberacing agent as a nucleating agent with data to show that carbon dioxide liberating agents alone fail to produce low density foams.
It is the objective of the present invention to pro-vide a low density foamed polymeric sheet, based on a poly-alkenyl aromatic polymer, that has multiaxial flexibility.
Another objective is to provide an improved process wherein polyalkenyl aromatic polymers can be formed into foamed sheets tha~ have multiaxial flexibility as extruded or after loss of blowing agent.
SUMMARY OF THE INVENTION
The present invention relates to a low density foamed polymeric sheet comprising a polyalkenyl aromatic polymer having dispersed therein about 0.1 to 5.0% of a polyolefin resin, based on said polymer, said foamed sheet having multiaxial flexibility.
The present invention also relates to a process for preparing a foamed polymeric sheet by extruding a composi-tion comprising:
(A~ a polyalkenyl aromatic polymer, (B) a volatile blowing agent, and ~C) a nucleating agent, as a melt under pressure through a sheet die into a zone of low pressure to permit foaming of said composition, the improvement which comprises having present in said composition about 0.1 to 5~ by weight of a polyolefin resin based on said alkenyl aromatic polymer, providing a foamed sheet having multiaxial flexibility.
~ILZ96~
PREFERRED EMBODIME~TS
The polyalkenyl aromatic polymers are prepared from alkenyl aromatic monomers selected from the group consist-ing of styrene, alpha methyl styrene, aralkyl styrene, e.g., ortho, para and meta substituted styrenes with alkyl groups such as methyl, ethyl, etc.; arhalostyrenes such as chloro and bromostyrenes, aralkylhalostyrenes, e.g., para-methyl~ortho, chloro or bromo styrene. Such polymers have a molecular weight of about 100,000 to 600,000 The polyolefin resins are those prepared by conven-tional polymerization having a molecular weigh~ of at least about 20,~00, pre~erably ~0,000 to 500,000 and a melt index (gm/10 mm) of about 0.1 to 55, preferably about 0.15 to 15 with a density of about 0.890 to .967 gms/cu.cm. Such resins are available from Union Carbide, New York, N. Y., and Northern Petrochemical Inc., Des Plaines, Illinois.
Conventionally, the nucleating agents are made up of -two materials which react to form carbon dioxide and water.
The two materials are normally used in approximately equivalent amounts. As the carbon dioxide libera~ing materials there can be used ammonium, alkali and alkaline earth carbonates or bicarbonates, e.g., ammonium bicarbon-ate, sodium bicarbonate, sodium carbonate, potassium bi-carbonate, calcium carbonate. The other material is an a~id or acid-reacting salt, preferably solid~ which is sufficiently strong to liberate the carbon dioxide from the carbonate cr bicarbonate. Generally, the acid has at least 3.0 milliequivalents of acidic hydrogen, and prefer-ably at least 10.0 miliequivalents, per gram. The acid can be organic or inorganic. Suitable acidic materials include boric acid, sodium dihydrogen phosphate, fumaric acid, malonic acid, oxalic acid, citric acid, tartaric acid~ potassium acid tartrate, chloro-acetic acid, maleic acid, succinic acid and phthalic acid. In place of the anhydrous acids or salts there can be used the solid hy-drates, e.g., oxalic acid dihydrate and citric acid mono-~2~6~) hydrate.
The nucleating agents can be solid inert materials such as talc, wax or hydrated salts or silicates used in finely divided form. The nucleating agents are used in amounts of about 0.2 to 10% by weight based on the poly-alkenyl aromatic polymer, preferably about 0.2 to 2% by weight is used. The nucleating agents are conventionally dry blended with the polymer prior to extrusion.
As the volatile blowing agent liquid, there can be used aliphatic hydrocarbons boiling between 10 and 100C.
and preferably between 30 and 90C., e.g., petroleum ether (containing primarily pentane or hexane or a mixture of these hydrocarbons), pentane, hexane, isopentane, cy-clohexane, cyclopentane, pentadiene and neopentane. Other volatile liquids include methanol, ethanol, methyl ace-tate, ethyl acetate, butane, acetone, methyl formate, ethyl formate, dichloroethylene, perchloroethylene, dichloro-tetrafluoroethane, isopropyl chloride, propionaldehyde, diisopropyl ether, dichlorodifluoromethane, a mixture of pentane with 5 to 30% of methylene chloride or other vola-tile lower halogenated hydrocarbon. Trichlorofluoro-methane, methyl chloride, ethyl chloride and other fluoro-carbons of khe Freon type may be used. Gases such as CO2 and N2 have also been found to be suitable blowing agents.
The blowing agent is used in amounts of about 2 to 20% by weight based on the polyalkenyl aromatic polymer.
One suitable apparatus for extrusion of said poly-alkenyl aromatic polymers containing carbon dioxide dis-persed therein under pressure through a die into a zone of low pressure is disclosed in U. S. Patent 3,451,103.
The extrusion apparatus consists of an extruder (preferably a single screw extruder) which contains three separate functional zones or sections. The first or plas-ticating zone of the extruder melts and delivers the melted resin to the second zone at a high temperature and pressure. The structure and design of the screw in the first zone may take a wide variety of forms, but typically 96~) consists of a constant pitch screw which increases in root diameter in the downstream direction. Heating means are usually included in the first zone to assist in melting the resin. If desired, the first zone may consist of two S elements, as for example by having a plasticating extruder arranged in tandem with the plasticating zone of a second extruder and delivering melted resin thereto.
The screw in the second zone may take a wide variety of forms, but usually is a constant pitch screw which has a constant root diameter. In addition, the root diameter in the second zone is usually either identical with or slightly smaller than the root diameter at the discharge end of the first zone. The second or injection zone is provided with specially designed means for injecting liquid substances into the melted resin. It is preferred to employ a plurality of such injection means and to have them symmetrically disposed about the chamber wall. The injection means employed are capable of injecting the liquid into the extruder at a pressure substantially higher than the pressure of the mel-ted resin, e.g., at a pressure of at least about 500 p.s.:i. higher than the pressure of the melted resin. The injection means also include an element adapted to seal the liquid delivery orifice when li~uid is not being injected into the melted resin. This feature prevents the melted resin from flow-ing into and plugging the orifice of the injection means.
The sealing element preferably consists of ta) a discharge orifice whose feed inlet terminates in a valve face, (b) a cooperatively functioning valve which is adapted to seat against and seal the valve face of the orifice, (c) a first and fixed pressure means acting upon and urging the valve into seated relationship with the valve face, and (d) a second pressure means acting upon the valve and urging it out of seated relationship with the valve face;
said second pressure means being responsive to and actu-ated by the pressure of the liquid within the injection means. To prevent melted resin ~rom flowing into the in-jection means, the fixed pressure means urging the valve ~2~6~
to seat against and seal the valve face should be preset to a pressure above that developed within the resin in the second zone of the extruder.
The third zone of the extruder performs two functions.
First, the pressure on the melted resin is increased to the level required to express the resin through the die.
Second, the melted resin is cooled (or in some circumstances heated) to substantially the temperature at which it will leave the die. To properly cool (or heat) the resin, at least the aft section of the third zone should include external heat transfer means. Depending upon the length of the second zone of the extrusion apparatus, it is sometimes desirable to maintain the mixture of melted resin and liquid at a relatively high temperature in the fore section of the third zone. In this event, external heating means may be provided to heat the chamber wall oE the fore section of the third zone.
In addition, the root diameter of the screw may be increased in the fore section of the third zone so that frictional heat will be developed within the resin. In this event, however, the root diameter is preferably subsequently decreased in the aft section of the third zone.
The die affixed to the extrusion apparatus may be of any design presently used in extruding thermoplastic resins.
Scores of suitable dies are known and reported in the art.
The three zones described can be separate extruders as practiced in the art.
The extrusion can be carried out at tempera-tures of about 135 to 160C. for the extrusion temperature at the die. Pressures for extrusion generally are greater than 30 1000 psi and range 1700 to 3700 psi with the blowing agent being injected at pressures at least about 500 psi higher than the pressure of melted resin in the extruder. About
2 to 20~ by weight of liquid blowing agent based on the polymer is injected into the melt depending on the density desired in the foam sheet which can range from about 0.008 to 0.16 gm/cm3. The blowing agent can be incorporated 6~
by dry blending the blowing agent with the polyalkenyl aro-matic polymer prior to extrusion in amounts of about 2 to 20% by weight based on the polymer. The blowing agent be-comes dispersed in the polymer melt under pressure. The melt containing the blowing agent is extruded from a flat sheet die or an annular die over an annular mandrel using conventional equipment commercially available. The sheet expands freely under atmospheric pressures as a lower pressure. Conventionally the sheet extruded from the die continuously has a longitudinal or machine direction di-mension and a wid.h or transverse dimension along with thickness or a gauge dimension.
The sheet can have a thickness of about 1 to 500 mils (0.025 to 12.5 mm.) with a cell size less than about 0.5 mm. ranging from about 0.005 to 0.5 mm. The cells under magnifaction have a unique cell structure wherein the cell wall has a generally uniform thickness with a uniform dis-tribution of cells. It is believed that the foamed sheet of the present invention, having unexpectedly uniform cell thickness, provide a foamed sheet with greater flexibility and less brittleness than prior art foamed sheets. The foamed sheet of the present invention has proven to have high utility as a ~rapping sheet or interlayer sheet ~or packaging, e.g., fruit in that it can be wrapped around the fruit without splitting and retain its crushable foam prop-erties to protect the fruit. Such sheet can be used broadly as a wrapping and cushioning sheet or con~ainer for packaging fragile items of commerce in general.
` Foamed sheets as extruded have the blowing agent con-tained in the cells which will diffuse out of the cells on storage. Generally, sheeting having compatible or soluble blowing agents such as hydrocarbons or fluorocarbons, are stored for a number o` days to allow the blowing agen. to diffuse out the sheet before commercial use as formed trays, egg cartons, etc., so that they will have the re-quired rigidity.
An accelerated test has been developed wherein the sheet is held at 70C. for 48 hours which will simulate .
2~6~0 such an aging period. The sheet can then be tested.
The sheet can be easily treated for flexibility by ASTM Test (D-2167-63T) which tests folding endurance.
Here, the sheet is folded repeatedly on itself through an 180 angle until the sheet parts, splits or cracks. It has been found unexpectedly that the foamed sheet of the present invention, having the polyolefin resin present, can be folded repeatedly on itself at least about 10 times without splitting or cracking, whereas sheets blown with hydrocarbons or fluorocarbons alone will crack on folding 180 having essentially no folding endurance.
A very simple test to determine the flexibility of the foam is to wad, roll or crush the sheet into a ball. If the sheet does not split or crack~ the sheet is considered flexible and will have the utility needed as an overwrap or formed sheet for packaging.
The polyolefin resins are resins of an olefin-monomer containing two to six carbon atoms. The preferred monomers are ethylene and propylene and are unsaturated hydrocarbons of the type CnH2n. Low de.~sity polyethylene generally has a density of about 0.910 to 0.925 ~ms/cm3; medium density polyethylene about 0.926 to 0.940 gms/cm3 and high density polyethylene about 0.941 to 0.967 gms/cu3. Propylene gen-erally has a density of about 0~890 to b . 908 gms~cm3. The amount of polyolefin resins found effective in preparing polyalkenyl aromatic foamed polymeric sheet is about 0.1%
to 5.0% more preferably about 0.3% to 3.0% and most prefer-ably about 0.5% to 2.5%, based on the polyalkenyl aromatic polymer.
The following examples zre set forth to illustrate the best mode of the present invention to those skilled in the art and do not limit the scope of the invention.
A sheet of foamed polystyrene was prepared employing an apparatus as disclosed in U. S. Patent 3,451,103. The barrel of the extruder was 2.5" (6.25 cm) in diameter and 120" (300 cm) long. Zone 1 is 50" (125 cm) long, Zone 2 is 28" (70 cm) long and Zone 3 is 42" (105 cm) long. The screw helical land has a constant pitch throughout its 6~
- g - C-08-12-0403 entire length. In Zone 1, .he first 7.5 L/D section of the screw has a root diameter of 1.76" (4.4 cmj; the second 5 L/D section of the screw has a root diameter which in-creases from 1.76" to 2.16" (4.4 cm to 5.4 cm); the third 7.5 L/D section of the screw has a root diameter of 2.16"
(5.4 cm~. Zone 2 has a constant root diameter of 2.16"
~5.4 cm). In Zone 3, the first 7 L/D screw section has a root diameter of 2.25" (5.625 cm) and a final 10 L/D screw section having a root diameter of 2.00" (5.0 c~). A sheet die of conventional construction was attached to the ex-truder to form a sheet of about 60 mil (1.5 mm).
Polystyrene pellets were dry blended with low density polyethylene (density about 0.92) pellets and a nucleating agent (sodium bicarbonate-and citric acid mixed on a 70/30 weight ratio of bicarbonate to citric acid) to form a dry blend containing 0.5~/O by weight nucleating agent and 0.5% by weight polyethylene, the remaining 99% being polystyrene.
The dry blend was fed to Zone 1 of the extruder at a rate of 54 5 kgs/hr. and is melt extruded with melt enter-ing Zone 2 at a temperature about 450F (232C.) under apressure of about 2700 psi (1.9 x 1o6 kgs/m2). Liquid tri-chlorofluoromethane is injected in Zone 2 into the melt stream a~ a pressure of about 3100 psi, (2.2 x 106 kgs/m2) at a rate of about 6.8 kgs per hour becoming dispersed in the melt in Zone 2. The melt enters Zone 3 at the tempera-tures and pressures of Zone 2 and is maintained at those conditions for about 17" (42.5 cm) of travel in Zone 3, then cooled by proper jacket temperatures to 310F. (154C.) over the last 25" (62.5 cm) of travel in Zone 3 leaving Zone 3 at about 2500 psi (1.75 x 106 kgs/m2~. The polystyrene melt is passed through a screen and breaker plate assembly and enters the die at about 1500 psi (1.05 x 106kgs/m2). The sheet is extruded as a 60 mil sheet (1.5 mm) having a dens-ity of about 2 lbs/ft. (32 kgs/m ). The cells of the foam sheet are less than 10 mil (.25 mm).
The sheet was tested by treating in a circulating air oven at 70C. ~or 48 hours wherein the foamed sheet comes into equilibrium with air, i.e., the cells become essen-tially air filled. This test simulates air agir.g at ambient 96~() ~ te~.peratures for a period of 60 days for commercially blown foam sheets before used by fabricators wherein the blowing agent is essentially replaced by air.
The sheet was found to be flexible having a fold en-durance under ASTM D-2167-63T of greater than 10 folds when repeatedly folded back and fourth through an angle of 180 on itself. The film was found to be crushable when wadded into a ball without splitting or cracking. Simple trays were formed as fruit containers having a depth of about one inch. The trays could be folded 90 without splitting.
Example 1 was repeated using no polyethylene in the dry blend. The sheet was tested as in Example 1 and found to be brittle in that the sheet could not be folded on itself through 180 and could not be wadded into a ball without splitting. Trays formed from the sheet were brittle, i.e., they could not be folded 90 without split-ting.
Exmple 1 was repeated using ].. 0, 1.5, 2.0, 2.5, 3.0 and 5.0~ low density polyethylene in the dry blend. The sheets were tested as in Example 1. All sheets were found to be flexible in the îold test taking more than 10 folds without splitting and were also crushable without cracking or splitting.
Tensile strength was also run on the sheets of Ex-amples 1-8.
.
., ~9 6'~
~ C-08-12-0403 TABLE I
% Tensile P.E. Strength* Flexibility 0 22.6 brittle 0.5 29.2 flexible 1.0 38.8 flexible 1.5 4~.5 flexible 2.0 45.7 flexible 2.5 43.8 flexible
by dry blending the blowing agent with the polyalkenyl aro-matic polymer prior to extrusion in amounts of about 2 to 20% by weight based on the polymer. The blowing agent be-comes dispersed in the polymer melt under pressure. The melt containing the blowing agent is extruded from a flat sheet die or an annular die over an annular mandrel using conventional equipment commercially available. The sheet expands freely under atmospheric pressures as a lower pressure. Conventionally the sheet extruded from the die continuously has a longitudinal or machine direction di-mension and a wid.h or transverse dimension along with thickness or a gauge dimension.
The sheet can have a thickness of about 1 to 500 mils (0.025 to 12.5 mm.) with a cell size less than about 0.5 mm. ranging from about 0.005 to 0.5 mm. The cells under magnifaction have a unique cell structure wherein the cell wall has a generally uniform thickness with a uniform dis-tribution of cells. It is believed that the foamed sheet of the present invention, having unexpectedly uniform cell thickness, provide a foamed sheet with greater flexibility and less brittleness than prior art foamed sheets. The foamed sheet of the present invention has proven to have high utility as a ~rapping sheet or interlayer sheet ~or packaging, e.g., fruit in that it can be wrapped around the fruit without splitting and retain its crushable foam prop-erties to protect the fruit. Such sheet can be used broadly as a wrapping and cushioning sheet or con~ainer for packaging fragile items of commerce in general.
` Foamed sheets as extruded have the blowing agent con-tained in the cells which will diffuse out of the cells on storage. Generally, sheeting having compatible or soluble blowing agents such as hydrocarbons or fluorocarbons, are stored for a number o` days to allow the blowing agen. to diffuse out the sheet before commercial use as formed trays, egg cartons, etc., so that they will have the re-quired rigidity.
An accelerated test has been developed wherein the sheet is held at 70C. for 48 hours which will simulate .
2~6~0 such an aging period. The sheet can then be tested.
The sheet can be easily treated for flexibility by ASTM Test (D-2167-63T) which tests folding endurance.
Here, the sheet is folded repeatedly on itself through an 180 angle until the sheet parts, splits or cracks. It has been found unexpectedly that the foamed sheet of the present invention, having the polyolefin resin present, can be folded repeatedly on itself at least about 10 times without splitting or cracking, whereas sheets blown with hydrocarbons or fluorocarbons alone will crack on folding 180 having essentially no folding endurance.
A very simple test to determine the flexibility of the foam is to wad, roll or crush the sheet into a ball. If the sheet does not split or crack~ the sheet is considered flexible and will have the utility needed as an overwrap or formed sheet for packaging.
The polyolefin resins are resins of an olefin-monomer containing two to six carbon atoms. The preferred monomers are ethylene and propylene and are unsaturated hydrocarbons of the type CnH2n. Low de.~sity polyethylene generally has a density of about 0.910 to 0.925 ~ms/cm3; medium density polyethylene about 0.926 to 0.940 gms/cm3 and high density polyethylene about 0.941 to 0.967 gms/cu3. Propylene gen-erally has a density of about 0~890 to b . 908 gms~cm3. The amount of polyolefin resins found effective in preparing polyalkenyl aromatic foamed polymeric sheet is about 0.1%
to 5.0% more preferably about 0.3% to 3.0% and most prefer-ably about 0.5% to 2.5%, based on the polyalkenyl aromatic polymer.
The following examples zre set forth to illustrate the best mode of the present invention to those skilled in the art and do not limit the scope of the invention.
A sheet of foamed polystyrene was prepared employing an apparatus as disclosed in U. S. Patent 3,451,103. The barrel of the extruder was 2.5" (6.25 cm) in diameter and 120" (300 cm) long. Zone 1 is 50" (125 cm) long, Zone 2 is 28" (70 cm) long and Zone 3 is 42" (105 cm) long. The screw helical land has a constant pitch throughout its 6~
- g - C-08-12-0403 entire length. In Zone 1, .he first 7.5 L/D section of the screw has a root diameter of 1.76" (4.4 cmj; the second 5 L/D section of the screw has a root diameter which in-creases from 1.76" to 2.16" (4.4 cm to 5.4 cm); the third 7.5 L/D section of the screw has a root diameter of 2.16"
(5.4 cm~. Zone 2 has a constant root diameter of 2.16"
~5.4 cm). In Zone 3, the first 7 L/D screw section has a root diameter of 2.25" (5.625 cm) and a final 10 L/D screw section having a root diameter of 2.00" (5.0 c~). A sheet die of conventional construction was attached to the ex-truder to form a sheet of about 60 mil (1.5 mm).
Polystyrene pellets were dry blended with low density polyethylene (density about 0.92) pellets and a nucleating agent (sodium bicarbonate-and citric acid mixed on a 70/30 weight ratio of bicarbonate to citric acid) to form a dry blend containing 0.5~/O by weight nucleating agent and 0.5% by weight polyethylene, the remaining 99% being polystyrene.
The dry blend was fed to Zone 1 of the extruder at a rate of 54 5 kgs/hr. and is melt extruded with melt enter-ing Zone 2 at a temperature about 450F (232C.) under apressure of about 2700 psi (1.9 x 1o6 kgs/m2). Liquid tri-chlorofluoromethane is injected in Zone 2 into the melt stream a~ a pressure of about 3100 psi, (2.2 x 106 kgs/m2) at a rate of about 6.8 kgs per hour becoming dispersed in the melt in Zone 2. The melt enters Zone 3 at the tempera-tures and pressures of Zone 2 and is maintained at those conditions for about 17" (42.5 cm) of travel in Zone 3, then cooled by proper jacket temperatures to 310F. (154C.) over the last 25" (62.5 cm) of travel in Zone 3 leaving Zone 3 at about 2500 psi (1.75 x 106 kgs/m2~. The polystyrene melt is passed through a screen and breaker plate assembly and enters the die at about 1500 psi (1.05 x 106kgs/m2). The sheet is extruded as a 60 mil sheet (1.5 mm) having a dens-ity of about 2 lbs/ft. (32 kgs/m ). The cells of the foam sheet are less than 10 mil (.25 mm).
The sheet was tested by treating in a circulating air oven at 70C. ~or 48 hours wherein the foamed sheet comes into equilibrium with air, i.e., the cells become essen-tially air filled. This test simulates air agir.g at ambient 96~() ~ te~.peratures for a period of 60 days for commercially blown foam sheets before used by fabricators wherein the blowing agent is essentially replaced by air.
The sheet was found to be flexible having a fold en-durance under ASTM D-2167-63T of greater than 10 folds when repeatedly folded back and fourth through an angle of 180 on itself. The film was found to be crushable when wadded into a ball without splitting or cracking. Simple trays were formed as fruit containers having a depth of about one inch. The trays could be folded 90 without splitting.
Example 1 was repeated using no polyethylene in the dry blend. The sheet was tested as in Example 1 and found to be brittle in that the sheet could not be folded on itself through 180 and could not be wadded into a ball without splitting. Trays formed from the sheet were brittle, i.e., they could not be folded 90 without split-ting.
Exmple 1 was repeated using ].. 0, 1.5, 2.0, 2.5, 3.0 and 5.0~ low density polyethylene in the dry blend. The sheets were tested as in Example 1. All sheets were found to be flexible in the îold test taking more than 10 folds without splitting and were also crushable without cracking or splitting.
Tensile strength was also run on the sheets of Ex-amples 1-8.
.
., ~9 6'~
~ C-08-12-0403 TABLE I
% Tensile P.E. Strength* Flexibility 0 22.6 brittle 0.5 29.2 flexible 1.0 38.8 flexible 1.5 4~.5 flexible 2.0 45.7 flexible 2.5 43.8 flexible
3.0 38.4 flexible 5.0 36.6 flexible * ASTM Test D-638 (kg/cm2) The data indicates that tensile strength is optimum at about 1.5 to 2.0% consistent with flexibility.
Example 1 was repeated using 0.5%, 1.0%, 2.0%,,3.0%
and 5.0% of polypropylene resin having a density of about 0.902 gm/cm in the dry blend, The sheets were tested as in Example 1 and test data is sho~l in Table II.
%
Poly- Tensile pro~ylene Strength Flexibility 0 22.6 brittle 0.5 28.9 flexible 1.0 35.3 flexible 2.0 48.6 flexible 3.0 45.2 flexible 5.0 44.3 flexible Example 1 was repeated using 0.5%~ 1.0%, 2.0%, 3.0%
and 5.0% of high density polyethylene resin having a density of about 0.950 gms/cm3 in the dry blends. The sheets were tested as in Example 1 and the test data is shown in ~L~296~C~
- 12 - C-08~12-0403 Table III.
TABLE III
% High Density Tensile Polyethylene Strength F'lexibility 0 ' 22.6 brittle 0.5 33.5 flexible l.0 34.8 flexible 2.0 35.6 ' flexible 10 3.0 34.1 flexible 5.0 32.5 flexible Maximum tensile strength appears at about 2.0% of high density polyethylene in the foamed sheet.
Lower levels Gf polyolefin resins were also studied to determine their effect on flexibility. The amounts of polyolefin type and test results are shown in Table IV.
The sheet was prepared and tested as in Example 1.
TABLE IV
% by Densi~y Weight Tensile Polyolefin gm/cm~ Used Stren~th ' Fl'e'xibil'i'~x Polyethylene O.g20 0.1 24.8 flexible Polyethylene 0.950 0.1 25.7 flexible' Polypropylene O.gO2 0.1 26.2 flexible It is evident that amounts as low as 0.1% by weight based on the foamed sheet can be used to induce flexibil'ity with a positive gain in tensile strength.
,., ' . . . : .
Example 1 was repeated using 0.5%, 1.0%, 2.0%,,3.0%
and 5.0% of polypropylene resin having a density of about 0.902 gm/cm in the dry blend, The sheets were tested as in Example 1 and test data is sho~l in Table II.
%
Poly- Tensile pro~ylene Strength Flexibility 0 22.6 brittle 0.5 28.9 flexible 1.0 35.3 flexible 2.0 48.6 flexible 3.0 45.2 flexible 5.0 44.3 flexible Example 1 was repeated using 0.5%~ 1.0%, 2.0%, 3.0%
and 5.0% of high density polyethylene resin having a density of about 0.950 gms/cm3 in the dry blends. The sheets were tested as in Example 1 and the test data is shown in ~L~296~C~
- 12 - C-08~12-0403 Table III.
TABLE III
% High Density Tensile Polyethylene Strength F'lexibility 0 ' 22.6 brittle 0.5 33.5 flexible l.0 34.8 flexible 2.0 35.6 ' flexible 10 3.0 34.1 flexible 5.0 32.5 flexible Maximum tensile strength appears at about 2.0% of high density polyethylene in the foamed sheet.
Lower levels Gf polyolefin resins were also studied to determine their effect on flexibility. The amounts of polyolefin type and test results are shown in Table IV.
The sheet was prepared and tested as in Example 1.
TABLE IV
% by Densi~y Weight Tensile Polyolefin gm/cm~ Used Stren~th ' Fl'e'xibil'i'~x Polyethylene O.g20 0.1 24.8 flexible Polyethylene 0.950 0.1 25.7 flexible' Polypropylene O.gO2 0.1 26.2 flexible It is evident that amounts as low as 0.1% by weight based on the foamed sheet can be used to induce flexibil'ity with a positive gain in tensile strength.
,., ' . . . : .
Claims (26)
1. A low density foamed polymeric sheet compris-ing a polyalkenyl aromatic polymer having dispersed therein about 0.1 to 5.0% by weight of 2 polyolefin resin, based on said polymer, said formed sheet having multiaxial flexibil-ity.
2. A sheet of Claim 1 having a folding endurance of at least 10.
3. A sheet of Claim 1 having a density of about 0.5 to 10 lbs./cu.ft. (8 to 160 kgs./cu. meter).
4. A sheet of Claim 1 having a cell size averaging less than 0.5 mm.
5. A sheet of Claim 1 having a cell size averaging about 0.005 to 0.5 mm.
6. A sheet of Claim 1 having a thickness of 0.025 to 12.5 mm.
7. A sheet of Claim 1 therein said polyalkenyl aromatic polymer is formed from monomers selected from the group consisting of styrene, alpha methyl styrene, aralkyl styrene, arhalostyrene, aralkylhalo styrene and mixtures thereof.
8. A sheet of Claim 1 wherein said polyolefin resin has a density of about 0.890 to 0.967 gm./cu.cm.
9. A sheet or Claim 1 wherein said polyolefin resin has a molecular weight of about least 20,000 and is selected from the group consisting of polyethylene and polypropylene.
10. A process for preparing a foamed polymeric sheet by extruding a composition comprising:
(A) a polyalkenyl aromatic polymer, (B) a volatile blowing agent, and (C) a nucleating agent, as a melt under pressure through a sheet die into a zone of low pressure to permit foaming of said composition, there being present in said composition about 0.1 to 5.0%
by weight of a polyolefin resin, based on said alkenyl aromatic polymer, providing a foamed sheet having multi-axial flexibility.
(A) a polyalkenyl aromatic polymer, (B) a volatile blowing agent, and (C) a nucleating agent, as a melt under pressure through a sheet die into a zone of low pressure to permit foaming of said composition, there being present in said composition about 0.1 to 5.0%
by weight of a polyolefin resin, based on said alkenyl aromatic polymer, providing a foamed sheet having multi-axial flexibility.
11. A process of Claim 10, said sheet having a folding endurance of at least 10.
12. A process of Claim 10, said sheet having a density of about 0.5 to 10 lbs./cu.ft. (8 to 160 kgs./cu.meter).
13. A process of Claim 10, said sheet having a cell size averaging less than 0.5mm.
14. A process of Claim 10, said sheet having a cell size averaging about 0.005 to 0.5mm.
15. A process of Claim 10, said sheet having a thickness of 0.025 to 12.5mm.
16. A process of Claim 10 wherein said polyalkenyl aromatic polymer is formed from monomers selected from the group consisting of styrene, alpha methyl styrene, aralkyl styrene, arhalostyrene, aralkylhalo styrene and mixtures thereof.
17. A process of Claim 10 wherein said polyolefin resin has a density of about 0.890 to 0.967 gm./cu.cm.
18. A process of Claim 10 wherein said polyolefin resin has a molecular weight of at least 20,000 and is se-lected from the group consisting of polyethylene and poly-propylene.
19. A process of Claim 10 wherein said blowing agent is selected from the group consisting of propane, butane, pentane, hexane, cyclohexane, heptane, petroleum ether, fluorochlorohydrocarbons, chlorohydrocarbons, fluorohydrocarbons, C02 and N2 or mixtures thereof.
20. A process of Claim 10 wherein said nucleating agent is selcted from the group consis.ing of alkali and alkaline earth carbonates or bicarbonates used in combina-tion with a solid organic acid or acid-reacting salt.
21. A process of Claim 10 wherein said nucleating agent is selected from the group consisting of talc, sili-cates and wax or mixtures thereof.
22. A process of Claim 10 wherein said blowing agent is present in said composition in amounts of about 2 to 10% by weight based on said polymer.
23. A process of Claim 10 wherein said nucleating agent is present in an amount of about 0.2 to 10% by weight based on said polymer.
24. A low density, flexible, foamed polymeric sheet formed by the process of Claim 10.
25. A low density foamed polystyrene sheet having dispersed therein about 0.1 to 5.0% by weight of a poly-olefin resin based on the weight of said polymer, said polyolefin resin selected from the group consisting of polyethylene and polypropylene or mixtures thereof, said sheet having multiaxial flexibility.
26. A process for preparing a foamed polystyrene sheet by extruding a composition comprising:
(A) a polystyrene polymer and based on said polymer, (B) about 2 to 10% by weight of a volatile blowing agent, (C) about 0.2 to 10% by weight a nucleating agent, and (D) about 0.1 to 5.0% by weight of a polyolefin resin selected from the group consisting of polyethylene and polypropylene, into a zone of low pressure to permit foaming of said composi-tion, said foamed sheet having multiaxial flexibility.
(A) a polystyrene polymer and based on said polymer, (B) about 2 to 10% by weight of a volatile blowing agent, (C) about 0.2 to 10% by weight a nucleating agent, and (D) about 0.1 to 5.0% by weight of a polyolefin resin selected from the group consisting of polyethylene and polypropylene, into a zone of low pressure to permit foaming of said composi-tion, said foamed sheet having multiaxial flexibility.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US5475279A | 1979-07-05 | 1979-07-05 | |
| US054,752 | 1979-07-05 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1129600A true CA1129600A (en) | 1982-08-10 |
Family
ID=21993287
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA355,505A Expired CA1129600A (en) | 1979-07-05 | 1980-07-04 | Flexible polyalkenyl aromatic polymeric foamed sheet |
Country Status (2)
| Country | Link |
|---|---|
| AU (1) | AU6012280A (en) |
| CA (1) | CA1129600A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4981631A (en) * | 1989-03-31 | 1991-01-01 | The Goodyear Tire & Rubber Company | Process for making lightweight polyester articles |
-
1980
- 1980-07-04 AU AU60122/80A patent/AU6012280A/en not_active Abandoned
- 1980-07-04 CA CA355,505A patent/CA1129600A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4981631A (en) * | 1989-03-31 | 1991-01-01 | The Goodyear Tire & Rubber Company | Process for making lightweight polyester articles |
| AU632373B2 (en) * | 1989-03-31 | 1992-12-24 | Shell Internationale Research Maatschappij B.V. | Light weight polyester article |
Also Published As
| Publication number | Publication date |
|---|---|
| AU6012280A (en) | 1981-01-15 |
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